DC to DC converters of the resonant type often require the use of parallel-connected power switches to provide adequate current levels, and they require large input and output filters. In addition, parallel operation of switching devices has the problem of current sharing, and large filters are disadvantageous from a size and weight standpoint. A typical prior art converter is shown in FIG. 1, and discussed later.
The output voltage of any DC-DC converter can be regulated by varying either: a) the switching frequency of the converter with respect to the resonant frequency at a constant pulse width, b) the pulse width of the converter at a constant switching frequency in the so-called Pulse Width Modulation (PWM) method, or c) the voltage of a voltage-fed pre-regulator in order to stabilize the output voltage of the power supply.
Each of these methods has its disadvantages:
a) Varying the switching frequency of the converter with respect to the resonance frequency does not allow operation in Zero Voltage Switch (ZVS) and Zero Current Switch (ZCS) mode over the entire range of the frequency operation, thus decreasing efficiency and increasing noise and electromagnetic interference (EMI).
b) The PWM method suffers from turn-on and turn-off losses, and requires use of lossy snubbers across the switches. The turn-on and turn-off losses increase as the operating frequency of the converter is increased.
c) The topology that employs a voltage-fed pre-regulator receives an input voltage and produces a constant output voltage signal across output nodes xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d (FIG. 1), even if the input voltage to the power supply exhibits substantial voltage transients. This is why a configuration whereby feedback from the output that varies the input voltage and output load, in order to regulate the output voltage, is a preferable circuit configuration. However, this topology includes two separate, series connected stages: a boost converter and a series resonance converter. As a result, the overall efficiency decreases, since the overall efficiency is a multiplication of two efficiencies, that of the boost converter and that of the series half-bridge converter.
Various attempts have been made to address the disadvantages of existing converters (power supplies) by providing a constant frequency, multiphase, full-resonant mode DC to DC converter, suitable for use in low voltage, high current DC power distribution systems. For example, U.S. Pat. No. 4,533,986 to Jones discloses a compact electrical power supply, shown in FIG. 1. Jones""s converter includes a serial sequence of a DC to DC boost converter 4, followed by a capacitive energy storage bank 6, and a half-bridge series resonant converter 8. Half-bridge converter 8 includes a transformer 10, an output rectifier and filter 12, a load 14, and a feedback module 15. Boost converter 4 increases the voltage available from a DC source (typically while presenting an inductive load thereto). Boost converter 4 includes an inductor Lboost connected to one input terminal of converter 4, and a solid state switch Qboost that is serially connected with Lboost between the input terminals of converter 4. A series switching diode Dboost is connected between Lboost and capacitive energy storage bank 6, which includes three capacitors CS connected in parallel to the output of the boost converter.
Conduction of Qboost causes current to flow in inductor Lboost and electrical energy to be stored in the inductor""s magnetic field and in switching diode Dboost. When Qboost is not conducting, Dboost is forward biased in the direction to allow the energy stored in inductor Lboost to be transferred in the form of an induced current to the boost converter output.
The capacitive energy in energy storage bank 6 is transferred to half-bridge series resonant converter 8. Half-bridge series resonant converter 8 is constructed from two solid state switches Q1 and Q2 that are serially connected to each other across capacitive energy storage bank 6, and an output transformer 10 having it""s primary winding T1 connected to the common output of switches Q1, Q2. Half-bridge series resonant converter 8 supplies the energy received from capacitive energy storage bank 6 to load 14 through output rectifier and filter 12. Half-bridge series resonant converter 8 is operated by controlling the gates of Q1 and Q2, through feedback module 15, causing alternate conduction of either Q1 or Q2 through primary winding T1 to a common terminal A of a series resonant circuit comprising two resonant elements Lr and Cr. This produces a pulse input to primary winding T1 of transformer 10, and causes energy to be supplied via a secondary winding T2 of transformer 10, and via output rectifier and filter 12 to load 14. The boost circuit parameters are selected to produce a continuous current flow in the Lboost inductor under normal load conditions.
The switching rates of solid state switches Q1 and Q2 always exceed the limits of human audibility, and are consistent with tolerable switching losses in the Qboost solid state switch, and with a minimum size for inductor Lboost, while achieving a significant voltage boost for increased energy storage in capacitive energy storage bank 6.
Regulating means in the form of a line regulator 30 are provided in boost converter 4, responsive to the voltage stored in CS, for adjusting the duty cycle of Qboost to insure the average stored voltage remains constant. Regulation of the output voltage occurs by adjusting the output voltage of the boost converter, or by adjusting the switching rate of Q1 and Q2 in relation to the resonant frequency of the series resonant elements Lr and Cr.
The disadvantages of the DC-DC converter disclosed by Jones, and of all substantially similar converters or power supplies include: a) boost converter 4 affects the entire input voltage, causing insufficient efficiency, especially on low input voltage conditions; b) series half-bridge resonant converter 8 has high RMS currents at both input and output, causing I2R losses and requiring the usage of large filter capacitors, in both input and output; c) resonant converter 8 has large switching losses because of the apparent absence of Zero Voltage and Zero Current switching, thus reducing the overall efficiency; and, d) the apparent absence of Zero Voltage and Zero Current switching results in a great deal of noise and EMI, requiring the addition of large filter elements in order to meet accepted standards.
There is thus a widely recognized need for, and it would be highly advantageous to have, a constant-frequency, multiphase, full-resonant mode, DC to DC converter (power supply), suitable for use in low voltage, high current DC power distribution systems, that does not suffer from the disadvantages of prior art systems listed above.
The present invention is of a novel, high efficiency DC-DC power supply, and of a method to operate it to increase its efficiency. Specifically, the present invention is of a low voltage, high current, half-bridge, series-resonant, multiphase, DC-DC power supply. The present invention discloses a series-resonant switching DC-DC power supply that includes a combination of a boost type pre-regulator (hereafter simply xe2x80x9cpre-regulatorxe2x80x9d) and a xe2x80x9cbase-subxe2x80x9d half-bridge series-resonant converter unit (hereafter xe2x80x9cbase-sub converter unitxe2x80x9d). The base-sub converter unit includes two (a xe2x80x9cbasexe2x80x9d and a xe2x80x9csubxe2x80x9d) serial half-bridge converters, connected in parallel to the pre-regulator, and having outputs that are combined to produce the required output power. This unique topology differentiates the base-sub converter unit of the present invention from prior art series-resonant converters. The power supply has inherently a Zero Voltage and a Zero Current switching feature. The power supply of the present invention provides improved ability for high current and low voltage operation, without the necessity of operating in parallel mode, so that the problem of current sharing is avoided. The pre-regulator output voltage is the sum of two voltages: an input voltage input to the pre-regulator, typically varying between 36-75V, and an additional (xe2x80x9cDeltaxe2x80x9d) voltage, whose amplitude varies (is adaptive) in response to the input voltage, in order to insure that the pre-regulator output voltage is fixed and stable. In other words, the pre-regulator performs an xe2x80x9cadaptive conversionxe2x80x9d on the input voltage in a way that provides a Delta voltage that complements the input voltage to obtain the required (and substantially constant) pre-regulator output voltage. For example, if the input voltage is 36V, and the required pre-regulator output voltage is 80V, the xe2x80x9cDeltaxe2x80x9d is 44V. If the input voltage is 75V, for the same 80V pre-regulator output voltage the xe2x80x9cDeltaxe2x80x9d is 5V. This adaptive use of the pre-regulator is the main reason for obtaining a significant increase in its efficiency, as well as in that of the entire power supply. The output of the pre-regulator feeds the input of the base-sub converter unit. The output current of the base converter has a 90xc2x0 output current phase shift relative to the output current of the sub converter. The 90xc2x0 phase shift between the two currents results in very low RMS currents in both the input and output of the power supply, and this greatly decreases I2R losses. In addition, because the base and sub converters are operating both in Zero Voltage and Zero Current switch mode, there are no switching losses. The innovative result here is that because the efficiency of each of the power supply sections (the boost type pre-regulator and the base-sub converter unit) is very high, the overall efficiency is also very high, typically about 91% overall. For example, the measured efficiency of the boost pre-regulator is around 98%, and that of the base-sub converter unit is 93%, thus the 91% overall efficiency.
According to the present invention there is provided a power supply comprising: a boost type pre-regulator having a pre-regulator input voltage and a pre-regulator output voltage and configured to perform an adaptive conversion on the pre-regulator input voltage, a base-sub converter unit that includes a base converter and a sub converter, the unit connected to the pre-regulator through a parallel connection of each of the base and sub converters, the base-sub converter unit receiving as input the pre-regulator output voltage and configured to provide a phase shifted combined output, and control means for controlling the pre-regulator and the base-sub converter unit, whereby the adaptive conversion by the pre-regulator, combined with the phase shifted combined output of the base-sub converter unit, provide a highly efficient power conversion by the power supply.
According to the present invention there is provided, in a power supply, a method for obtaining a low ripple, low loss output comprising: providing a boost type pre-regulator having a pre-regulator input voltage and configured to perform an adaptive conversion on the pre-regulator input voltage, a base-sub half-bridge series resonant converter unit that includes a base converter and a sub converter, the base-sub converter unit connected to the pre-regulator through a parallel connection of each of the base and sub converters, performing the adaptive conversion on the pre-regulator input voltage, the adaptive conversion outputting a stable and substantially constant pre-regulator output voltage to the base-sub converter unit, and obtaining a phase shifted combined output from the base-sub converter unit, the phase shifted output characterized by a low ripple, whereby the combined action of the pre-regulator and the base-sub converter unit results in a low power loss, low switching loss, high efficiency power conversion in the power supply.